What Happens When a Submarine Hits Crush Depth?

I mean, does the sub sink because internal valves, pipes, etc. start leaking and fill the sub with water?
Or does the hull implode in one massive event? I would hope (for the crew’s sake), that the event is quick and painless…instead of guys struggling to close watertight doors, tighten leaking valves, etc.
When WWII subs reaches crush depth (about 1000’), did anything float up? Or was everything confined in the hull?

There are two components to your question : what happens when a sub reaches its rated crush depth (i.e. the depth at which its manufacturers and engineers have determined it wouldn’t be safe to dive under) and its actual crush depth.

The first is, well, unsafe, but not necessarily the end. All WW2 subs were rated at a given max depth, but due to manufacturing differences it was almost impossible to know for sure just how far down they could really go. What with the whole war thing going on, some skippers tried their luck when there was a need for it. Some even came back up. Going too deep however stresses the whole hull unduly, and might also lead to fuel and air leaks (the fuel tanks and ballast tanks are installed between the outer, thick pressure hull and the thin habitacle hull. If the pressure hull “deflates”, the tanks are compressed between the two hulls, increasing the pressure in both the tanks, and the pipes).

Actual crush depth isn’t called *crush *depth for nothing. That’s when the outer hull finally snaps, immediately followed by the fuel and air tanks, as well as the inner hull which are not at all rated for such pressures. Basically, the subs crumbles like a tin can. We’re not talking leaks, we’re talking walls of water rushing in. The air trapped inside can either form a bubble at either end of the boat (in which the crew will slowly suffocate), or leak/blow up and let them all die by drowning. Neither is a particularly nice way to go.

As to the question of anything floating up, why not ? There’s a gaping hole in the submarine now, and the fuel lines are crushed, so at the very least we’re looking at an oil spill.

I saw something on the Discovery Channel that the pressure increase would cause the remaining air to heat up and cook the crew.

I recently read Black May, and it described on several occasions debris floating to the surface-typically fuel oil, but also body parts and buoyant sub pieces.

These Wikipedia links have a good description and images of what happens when a sub dives below its true crush depth.

Bob Ballard has described it as like being inside the cylinder of a diesel engine. The water rushing in at high pressure acts like a piston combusting everything inside the hull. It is know as telescoping where the hull implodes like closing a spyglass, the men don’t have time to drown.

Actually, most submarines have a strong pressure hull surrounded by a thin, non-watertight “skin” of steel that holds the ballast tanks, sonar sphere, etc.

Regarding the implosion of a submarine whose hull has ruptured, as others have stated, it is a catastrophic event. When I was on watch on a submarine, we used to speculate on what it would be like. :eek:

I’ve heard speculation that the implosion would be similar to what happens when you set off a bomb calorimeter, in which the extreme pressure rise would heat up the internal atmosphere to the point that combustible materials would be incinerated as the water rushed in.

For illustration purposes, it can be calculated that a hole in the submarine’s hull of just 1-foot diameter (at a depth of 800 feet) would fill the associated compartment in just a few seconds. Larger hull ruptures would flood the submarine essentially instantaneously.

Nothing to add, except that the extended edition of the James Cameron undersea-adventure movie The Abyss has a pretty chilling depiction of a submarine loss in the first few minutes.

You’re right, of course. I mixed myself up, there. Sorry :o

In the case Scorpion, the after bulkhead failed and was pushed 50ft up into the hull.

Maybe a little offtopic, but I’ve wondered what happens if a sub is at almost maximum depth and a big storm wave, say 30m, passes overhead. Does the hull suddenly feel an overload, and is the “suddenly” part a factor?

About compression fires:

Not sure how this would work.

To be sure when you compress something it gets hot. I used to work in a scuba shop and the tanks would heat up quite noticeably when being filled (so we’d put them in a tank of water to keep them cooler and could get more air in). FWIW the reverse is also true. Decompression cools things down. Same scuba shop I overfilled a tank (long story but I was not as careless as that sounds) and the pressure relief valve let go. Once the tank was empty there was a block of ice around the relief valve.

Anyway, I can certainly see an implosion compressing and heating the air in the sub. That said this all would happen rather quickly. I think we are talking catastrophic failure here. So, implode, air heats up, ignites and is quenched by tons of water.

I would imagine the whole process would be very fast. For the sailors they would be simultaneously compressed by the air, set on fire, quenched by water and crushed by water…not to mention the submarine itself squishing them.

That’s a lot of bad shit to happen to one person all at once but what kills them? I suspect it is the sub crumbling around them and squishing them. I doubt any of the other effects have the time to do their thing.

Whatever the case bad news for the sailors. On the upside I think once it occurs death would probably be so fast as to be unnoticeable to them…I hope (which does not account for the time they get to think about their imminent doom as they sink).

Agreed - I doubt if in most cases it’s as much as a second between the heating and the quenching. The air doesn’t compress unless/until tons of water are rushing in to take (most of) its former place.

I doubt the hull crumples comprehensively. The instant it deforms enough to rupture somewhere, water rushes in and rapidly cancels the crushing force.

WWII fleet boats max designed depth was 400 feet. Keth O’Kane on the Tang went deeper. On trials he took the boat deeper until he blew a gasket in a flange. Brought the Tang back up, repaired the flange installed stronger bolts. Went deeper the next time until another leak developed, had that repaired and strenghtened. He kept it up until he could take the Tang down to 600 feet with no leaks.

He used going to 600 feet to escape depth charging because the Japanese knew the max operating depth of a US fleet boat was 400 feet. I an not absolutely sure but I believe he learned this trick from Mush Morton.

There is a book called Shadow Divers that goes into great detail about what happens in a sinking submarine. Its a really good read about diving on and in a sunken U-boat.

I wonder this too. I remember reading somewhere (perhaps in the book How to Plan a Safe Dive? It’s been a while since I looked at it) about waves causing potentially dangerous rapid pressure changes in divers tethered to non-floating structures, especially near the surface, but I don’t know how big a wave would have to be to to cause a problem with a submarine…

Based on what I have read in several books once you get a couple of hundred feet down you don’t feel any surface wave action regardless of what is going up upstairs.
Maybe Robby can comment.

<asking everyone>

So in reference to that other thread about explosions in subs and ships that sink, would the rapid compression detonate explosives on the sub (other than the depth charges with pressure triggers). It’s not just the high pressure, but the rapid rate of compression which would tend to exceed the maximum pressure reached when an equilibrium is reached.

I read"The Thresher Disaster" many years ago and distinctly remember the description was an implosion followed by an explosion as the diesel fuel on board detonated under the pressure. I’m not sure if this is true and would like to confirm if that is feasible or correct. I am aware that it was nuclear powered, but I remember the book stating they had diesel fuel for some reason

Nuclear subs have a backup diesel engine.